Angewandte
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Herein, we show that the resulting HBCy2 Shimoi
B, O, P, and N, where M = Rh or Ir) oriented in a near-
À
À
complexes, having a d-B H agostic interaction, allow for
coplanar fashion. Evidence for a significant [M]···H B bond-
ing interaction comes from a combination of solution- and
complete reversal of HBCy2 chemoselectivity preference,
thus enabling aldehyde hydroboration in the presence of an
alkene, while regenerating the k2-N,O-chelated starting
material. To our knowledge, hemilability and joint metal–
ligand cooperativity have not been used previously for both
the capture and subsequent chemoselective functionalization
of a borane molecule.
solid-state techniques. In the solution state, complexes 7 and 8
show characteristic H NMR resonances, in accordance with
1
their proposed structure. Strikingly, broad upfield resonances,
which sharpen on 11B decoupling, at d = À8.70 ppm (dd,
1JRh,H = 23.8 Hz, JP, H = 9.5 Hz) for 7 and d = À4.89 ppm (d,
3
3JP, H = 7.7 Hz) for 8, corroborate interaction of the metal with
The reaction of [{M(m-Cl)(h4-cod)}2] (0.5 equiv; M = Rh
or Ir; cod = 1,5-cyclooctadiene) and the sodiated phosphor-
amidate ligand salt Na[Xyl(N)P(E)(OEt)2] (1; 1 equiv)[11] at
room temperature provided the k2-N,O-coordinated com-
plexes [M{k2-N,O-Xyl(N)P(O)(OEt)2}(h4-cod)] (M = Rh (3),
Ir (4)) in 80 and 65% yield, respectively (Scheme 2). The
related phosphoramidothiolate complexes 5 and 6 were also
prepared by an analogous strategy in 92 and 81% yield. These
complexes were characterized by multinuclear NMR spec-
troscopy, mass spectrometry, X-ray diffraction, and elemental
analysis. Characteristic of metallacycle formation, the 31P{1H}
NMR spectrum showed a single downfield-shifted resonance,
for example, at d = 13.0 ppm (2JRh,P = 9.7 Hz) for 3 and d =
27.7 ppm for 4.
the B H entity.
Ring expansion of the parent four-
[13]
À
membered metallacycles 3 and 4 is also evidenced by
a change in the 31P{1H} NMR chemical shift to d = 22.1 ppm
(2JRh,P = 4.5 Hz; Dd = 9.1) for 7 and d = 34.3 ppm (Dd = 6.6)
for 8. 11B{1H} NMR spectroscopy additionally provided one
broad signal attributable to an oxygen-stabilized sp3-hybrid-
ized boron atom at d = 13.0 ppm (w1/2 = 573 Hz; Dd = À46.2;
d = 59.2 for free HBCy2)[14] for 7 and d = 14.6 ppm (w1/2
=
670 Hz; Dd = À44.6) for 8. An EI MS experiment also showed
a signal at m/z 645 (7) and 735 (8) with the correct isotope
pattern, though [MÀHBCy2]+ was identified as the base peak,
thus highlighting the weak degree of stabilization provided to
the borane by the phosphoramidate fragment in the gas
phase. Unexpectedly, treatment of the sulfur analogues 5 or 6
with HBCy2 (1 equiv) provided no reaction, as observable by
1H NMR spectroscopy, thus highlighting the importance of
=
the P O···B interaction.
The solid-state structure of complexes 7 and 8 was
established by single-crystal X-ray diffraction (complex 8:
Figure 1). Both complexes feature four-coordinate d8 metal
centers, supported by two olefinic groups of a 1,5-cod moiety,
the phosphoramidate nitrogen atom [Rh(1)–N(1) 2.118(1)
and Ir(1)–N(1) 2.097(2) ], and a bridging hydride of the
stabilized HBCy2 unit [Rh(1)–H(1) 1.70(2) and Ir(1)–H(1)
1.63(2) ]. It is notable that these crystals were of sufficient
quality to allow for the location and free refinement of H(1).
=
As expected for a P O···B interaction, the B(1)–O(1)
distance [1.529(2) and 1.519(2) for 7 and 8] is similar to
À
the dative B O bond length found for a family of ortho-
phosphine-oxide-substituted boranes (1.52–1.62 )[15] and yet
À
longer than the average B O distance observed for four-
coordinate borates [davg(B(1)–O(1)) (1.481 Æ 0.041) ].[16]
With regard to the OPN ligand framework, O(1)–P(1) and
N(1)–P(1) bond lengths of 1.519(1)/1.592(1) for 7 and
2
=
À
1.515(1)/1.602(1) for 8 suggest P O and P N bonds.
To better determine the location of the bridging hydride
[H(1)], density functional theory (DFT) calculations[17] were
performed on complexes 7 and 8. The computed geometries
are in good agreement with those observed crystallographi-
cally (see the Supporting Information), including the exper-
imental and computed B(1)–H(1) and M(1)–H(1) bond
lengths: 1.330/1.31(2) and 1.723/1.70(2) for 7 and 1.355/
1.41(2) and 1.733/1.63(2) for 8. The computed IR stretching
À
Scheme 2. Synthesis of the d-B H agostic complexes [M{k -N,H-Xyl-
(N)P(OHBCy2)(OEt)2}(h4-cod)] (M=Rh (7), Ir (8)), and 1H and
1H{11B} NMR spectra for 7 (400 MHz, C6D6, 298 K).
Having established a simple protocol for access to such
LX-chelated[10] metal complexes, we next focused our atten-
tion on the capture of dicyclohexylborane (HBCy2) by
complexes 3–6. Treatment of a solution of 3 or 4 in C6D6
with HBCy2 (1 equiv) resulted in the clean formation of the
four-coordinate complexes [M{k2-N,H-Xyl(N)P(OHBCy2)-
(OEt)2}(h4-cod)] (M = Rh (7) or Ir (8)), which feature six-
membered chelating LX amidoborane moieties formed from
À
frequency for the [M]···H B entity is also in excellent
agreement with that found experimentally: 2048/2043 cmÀ1
for 7 and 2034/2030 cmÀ1 for 8.[18]
In our hands, clear orange solutions of 7 or 8 in C6D6
became black on standing after 1 week. Analysis by 1H NMR
spectroscopy corroborated decomposition of the parent
species and formation of the tetrameric metal-bridged
À
hemilabile ring-opening of a [M] O bond (Scheme 2). Com-
plexes 7 and 8 are rare examples of six-membered genuine
metallaheterocycles,[12] consisting of six unique atoms (M, H,
3182
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Angew. Chem. Int. Ed. 2016, 55, 3181 –3186